WordPress Theme Search - What WordPress Theme Is That?
Blocksy - A lightningfast, innovative and supercharged WordPress theme
GPLDL - 1066 GPL-licensed WordPress themes & plugins for free
Readable WordPress Theme - The first Medium.com like WP theme with great readability
CloudSight - Image recognition API; send an HTTP request with an image, get a description of contents.
Neve - With an intuitive header builder, 100+ pre-designed sites, a LOT of WooCoomerce modules, global colors & a way to share saved templates across sites, Neve is a great choice for beginners & freelancers/agency owners alike.